Models aren't sold in at the same scale (hue) as toys are. Model companies release fewer of them, so they continue re-re-re-releasing old kits to keep a steady revenue.
Stores also want to have a steady revenue, so they order old kits, along with the new kits and will fill up their shelves with as much stock as they can hold.

Toy companies used to have year round production for their toylines. Everything they made that year would be restocked several times a year.
Toy retailers, like Walmart, found out that you could attract more customers by making it like an event though. IF you change up the shelves every quarter, you make people come into check their store more frequently. So at first toy companies started making two waves a year, and eventually started the quartlerly schedule we have today.

Because toys sell so much and there's only so much shelf/warehouse space available, retailers can only really handle to carry NEW toys. So this is why you don't see much old shit, unless it's some redeco or repack into the newest waves.
Toy stores like Kay Bee and TRU can handle year round stock, since they have a ton more space dedicated to toys, but toy makers have to cater to the whims of Walmart, since they're so big. And since Target/KMart have similar business models, there's more reason to not do year round waves anymore.

Also, ever notice that /toy/ used to shit on ToysRUS for having old stock sitting around? Like, six month old stock, sitting along with new waves and bitching that it never goes on sale. Somehow it also means it's a poor seller, despite toy stores purposely ordering that much stock to keep in place for that long. So public perception became negative for just having old stock, despite it being completely normal to see in the early 90s and before.